1. Field of the Invention
The present invention relates to a process for correcting noise level. It applies to image detectors of the type in which the acquisition of the images is performed by a matrix of photosensitive points. The process of the invention is more particularly aimed at reducing the perception of noise which is correlated between all the photosensitive points of one and the same row.
2. Discussion of the Background
Matrices of photosensitive points are commonly used in image acquisition techniques, where they make it possible to obtain digitized images. In these photosensitive matrices, the photosensitive points are produced in particular with the aid of techniques for depositing thin films of semiconductor materials. These photosensitive matrices make it possible to detect images contained in visible or near-visible radiation. It should be noted that they find a particularly beneficial application in the detection of radiographic images; to this end, it is sufficient to interpose a scintillator screen between the incident X-rays and the photosensitive matrix, so as to convert the X-rays into radiation within the band of wavelengths to which the photosensitive points are sensitive.
There are photosensitive matrices of large dimensions (for example 50 cm×50 cm), which may have up to several million photosensitive points or pixels. The photosensitive points form a network of rows and columns. A photosensitive matrix, of the type in which each photosensitive point comprises a photodiode cooperating with an interrupter element consisting of a switching diode, is described with its mode of operation as well as an embodiment thereof, in a French patent application No. 86/14058 (publication No. 2,605,116).
When the matrix is exposed to a luminous cue, quantities of charges are generated and accumulated by each of the photosensitive points, as a function of the intensity of the signal to which it has been exposed. Reading of these quantities of charges is carried out row by row. It consists in particular initially in transporting the charges accumulated by each of the photosensitive points of the row addressed, to columnwise reading circuits, by way of conductors parallel to the columns. Thereafter, multiplexing circuits then make it possible to transfer these quantities of charge, in the form of measurement values, to a data acquisition circuit where they are stored and processed.
Generally regardless of the mode of operation of the matrix, each measurement value is overlaid with a noise level, the origin of which is in particular the noise affecting the various electronic facilities involved in the reading, in particular the row addressing circuits and the columnwise reading circuits.
The term “noise” is understood to mean that the measurement value Vm, delivered by each photosensitive point, can only be reproduced with a certain error or statistical fluctuation characterized by its standard deviation.
It is well known that in image detectors of the type with photosensitive matrix, a non-zero noise density at frequencies below the row addressing frequency (the row frequency is the inverse of the time allocated to the reading of the photosensitive points of one and the same row), results in all the photosensitive points of this row being affected by the same noise: this noise is then referred to as “row-wise correlated noise” Bcl.
It is also well known that the eye is especially effective in detecting all the correlated phenomena in am image. This renders the row-wise correlated noise Bcl especially formidable since it is detectable even at very low levels. It is generally considered that, relative to uncorrelated noise Bnc whose level corresponds to the limit of perception by the eye, correlated noise Bcl is still perfectly detectable by the eye when it possesses a level of the order of 10 times lower than that of the uncorrelated noise.
To this problem which results from the rise in the capacity of perception of the eye to correlated fluctuations, a conventional solution is afforded by a noise correction method referred to as the “clamp” technique; in this method, the value VBcl of the correlated noise Bcl present in a given row is determined so as to subtract it from the measurement value Vm delivered by each of the photosensitive points of this row (Vm−Vbcl). For this purpose, this method consists in particular in reading a so-called dark value Vdk (which contains the row-wise correlated noise), delivered by a photosensitive point belonging to the row and deliberately left in the dark. This is achieved by protecting the photosensitive point from any exposure to a luminous cue; this point thus fulfills solely a correction function and it is referred to as a “corrector point” in the subsequent description. The other photosensitive points intended to be exposed to luminous cues with a view to detecting an image are referred to as “detector points”.
However, if the dark value Vdk of such a photosensitive point actually contains the cue of the row-wise correlated noise Bcl, it also possesses a certain level of uncorrelated noise Bnc; the dark value Vdk is in fact the quadratic sum of the correlated noise Bcl and uncorrelated noise Bnc. Additionally, with a view to providing a correction value exhibiting a reduced level of uncorrelated noise Bnc, more than one corrector point, i.e. n corrector points, are allocated to the function for correcting the row-wise correlated noise Bcl. Under these conditions, a correction value Vc obtained ultimately by averaging the dark values Vdk of these n corrector points is representative of the correlated noise level Bcl, whilst the larger the number n of corrector points, the greater will have been the statistical reduction in the uncorrelated noise Bnc. More precisely, the uncorrelated residual noise level is reduced by a factor equal to the root of the number n of corrector points (Vc=Bcl+Bnc/√n).
When the measurement value VM of each of the detector points of the row is corrected by the correction value Vc, the original row-wise correlated noise Bcl is eliminated, but it is replaced with some other noise Bcl′ injected through the correction itself. This noise Bcl′ is the uncorrelated residual noise which itself becomes row-wise correlated through the very fact that it is present in all the detector points. This new row-wise correlated noise Bcl′ therefore has an amplitude equal to that of the uncorrelated residual noise, i.e. Bnc/√n. Under these conditions, if one wishes to confer on this new correlated noise Bcl′ a level below that at which it is perceived, while complying with the rule according to which a correlated noise must be less than at least 10 times the uncorrelated noise, we must have √n=10, and hence n=100 (n being the number of corrector points of the row).
This solution produces good results when [sic] to the perception of noise by the eye, but it is extremely penalizing on account of the high number n (n>100) of corrector points which it requires for each row. Indeed, the high number of corrector points not only increases the cost, but it increases the bulk for one and the same area intended for image detection, since for each row, these 100 corrector detectors must be left in the dark.